In the fabrication of high temperature articles, particularly gas turbine engine components, there is a demand for techniques for bonding articles together by a process which will produce a high strength joint. Most preferably the bond joint will be defect free and have a strength comparable to that of the articles being joined.
The transient liquid phase bonding process described in U.S. Pat. No. 3,678,570 addresses this problem and describes a process for bonding articles together at constant temperature. Briefly, boron containing nickel foil is placed between the articles to be bonded and the assembly of articles and foil is forced together and heated to a temperature at which the boronized portion of the interlayer foil melts but the articles being joined do not melt. When the assembly is held at this essentially constant temperature for some period of time the boron will diffuse into the unmelted portions of the assembly and eventually solidification will occur at the elevated temperature when the localized boron content is sufficiently reduced by diffusion. However, problems have been encountered employing this process in the bonding of fine grain materials.
It is believed that the problems encountered are related to the more rapid diffusion of boron in fine grain materials than in coarse grain materials. Fine grain materials have a substantially greater volume of grain boundary than coarse grain materials, and boron has a much greater diffusion rate in grain boundaries than in material away from the boundaries.
The foils employed in the transient liquid phase bonding process are rich in boron, boron being a very effective melting point depressant, and are typically produced by diffusing boron into the surface of the foil, for example, according to the teachings of U.S. Pat. No. 3,753,794.